1. Field of the Invention
The invention relates generally to the field of semiconductor devices and, more particularly, to dissipating heat generated by the operation of such devices.
2. Description of Related Art
One goal of complementary metal oxide semiconductors (CMOS) in very large scale integration (VLSI) and ultra large scale integration (ULSI) is to increase chip density and operation speed. However, with increased chip density and operation speed, CMOS power consumption is also increased dramatically. It is expected that the power consumption of a high performance microprocessor will increase from several watts currently to approximately several hundred watts in the near future. The heat generated from this power consumption will raise chip temperature dramatically and degrade circuit performance and reliability. Therefore, reducing chip operation temperature is of great importance for current as well as future VLSI and ULSI technology.
To date, reduction of chip temperature is accomplished in two ways: 1) Lowering the power consumption, and 2) improving heat dissipation to the ambient environment. The first method is the preferred approach. A lowering of the power consumption is usually accomplished by scaling down the power supply voltage. The power consumption of integrated circuit chips has decreased from 5.0 volts several years ago to today's approximately 1.5 volts. However, lowering of the power supply voltage may impact negatively on the performance of the device. Because of the non-scaleability of the build-in voltage of a silicon junction, there is little room for further reduction of the power supply voltage below 1.0 volts if traditional technology is used. Thus, for high performance VLSI and ULSI circuits, further lowering of the power supply voltage may not be the most effective approach.
As indicated previously, the second approach to the reduction of chip temperature is through improved heat dissipation to the ambient environment. The heat dissipates mainly through the silicon substrate into a metal heat sink inside the package and through a metal interconnect system. This approach typically employs a heat sink/ground plan in physical contact with the silicon substrate. Some modern technologies, however, have eliminated the heat sink/ground plan in physical contact with the silicon substrate. One example is flip-chip technology wherein the chip is inverted so that the interconnect system lies on the underside of the chip rather than on the exposed top surface. These technologies encapsulate the silicon chip inside a package with epoxy material thus eliminating the contact between the silicon substrate and a heat sink. Instead, the metal interconnect system becomes the dominant heat dissipation path.
Heat dissipation through the interconnect system may be improved by increasing the total physical contact area to a heat source. A large effective physical contact area will reduce the thermal resistivity proportionally. In a typical chip design, the primary effective thermal contact to the transistor is provided by the diffusion or source/drain contact. The total source and drain physical contact area is, however, limited to a small percentage of the total chip size because other structures, such as an active channel, isolation, metal interconnect, and separation space, consume a much larger area of a given chip. Thus, the current design of the thermal contact area to the transistor (i.e., the area available to effectively dissipate heat generated by the transistor) is insufficient to dissipate the heat generated by the power consumption anticipated for future CMOS technology.